The present invention relates to a vehicle including an auxiliary power unit, and more particularly, to a control system for the auxiliary power unit.
Large tractors or trucks (e.g., semi-tractors) are commonly used to transport cargo within a trailer or container. Existing tractors in a tractor-trailer combination typically include a cabin that is conditioned by multiple mechanically driven vapor compression air conditioning systems. In most tractors, the air conditioning system and other components of the tractor and/or trailer are typically powered by a primary engine of the tractor when the vehicle is operating or moving. Some tractors include an auxiliary power unit that has a secondary engine supplying power to an auxiliary air conditioning system (e.g., compressor) and other components of the tractor when the primary engine is off. Some auxiliary power units also are used to charge tractor batteries (primary or auxiliary) when the primary engine is off.
Existing auxiliary power units are often overloaded when the units simultaneously provide power to the air conditioning system, the batteries, and other components of the truck due to the limited capacities of these auxiliary power units. FIG. 1 illustrates a typical auxiliary power unit load diagram of a prior art auxiliary power unit that has a maximum power output of approximately 5.1 kilowatts (7 horsepower) with two distinct loads—an alternator load and a compressor load. At time T0, the auxiliary power unit is started and the alternator load is applied to the auxiliary power unit. At time T1 (approximately 7 seconds) the alternator reaches a steady state load of approximately 2.9 kilowatts (4 horsepower).
As is common in conventional systems, the compressor load (e.g., approximately 3.7 kilowatts or 5 horsepower) is applied to the auxiliary power unit after and in addition to the alternator load at time T2 (e.g., about 40 seconds after the auxiliary power unit is started). The compressor has a startup load phase in which the load on the auxiliary power unit is increased to approximately 6.6 kilowatts (9 horsepower) because the peak compressor load of 3.7 kilowatts is added to the alternator load of 2.9 kilowatts that is still applied to the auxiliary power unit. The startup load phase is relatively brief (e.g., less than 5 seconds) and then begins its transition to a steady state load at time T3. As illustrated the time T3 is approximately 43 seconds after the auxiliary power unit has started (i.e., the compressor startup load phase is approximately 3 seconds long). The compressor achieves a steady state load of approximately 2.2 kilowatts (3 horsepower) at time T4, and as illustrated, the transition to steady state load lasts about 17 seconds (i.e., 60 seconds after the auxiliary power unit has started).
However, as illustrated in FIG. 1, when the alternator load and the compressor load are applied at the same time to the conventional auxiliary power unit at T3 (i.e., when the compressor transitions toward steady state load), the total load of approximately 6.6 kilowatts (9 horsepower) exceeds the load that can be handled by the auxiliary power unit. As a result of the finite load capability of the auxiliary power unit and the excessive, cumulative loads on the auxiliary power unit cause the alternator or the air conditioning system, or both, to operate at lower than desired thresholds (e.g., between time T2 to time T4).